FAILURE ANALYSIS OF MICROFABRICATED IRIDIUM ULTRAMICROELECTRODES IN CHLORIDE MEDIA

This study investigates the failure mechanisms for an electrochemical sensor consisting of an array of microlithographically fabricated irid ium ultramicroelectrodes (UME) on a silicon wafer. It has been noted t hat the use of these sensors during voltammetric determination of heav y metal ions in the presence of > 0.1 M Cl- results in loss of respons e and general failure of the sensor. The addition of Hg or Ag to the C l containing solution caused massive accumulations on the exposed Ir s urfaces, which were identified using auger scanning electron microscop y (SEM). Under analytical conditions this failure does not appear to b e due to migration of Cl- but to the penetration of Cl- through pin-ho les in the insulating layer and subsequent reaction with the Al interc onnect traces on the chip. Consequently, the Al traces dissolved and h ydrogen gas was evolved. This reaction generates a potential which is transmitted to exposed Ir and consequently metal ions present in the s olution are then reduced onto these surfaces. Several insulating layer s, SiO2 (5000 Angstrom) and Si3N4 (1500, 2500 and 5000 Angstrom) were also used, but failure occurred with all. It is critical that either s ub-micron sized pin-holes during the fabrication process be better con trolled or that the Al interconnect traces be replaced by Au. The use of Au however may still allow these unwanted micron-sized UMEs to inte rfere with the electrochemical analysis. (C) 1998 Elsevier Science S.A . All rights reserved.